Guarantee of Registered Securities (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
CURRENT ASSETS :
Cash and cash equivalents	$ 408	$ 291	$ 323	$ 272
Short-term investments	352	346
Receivables, net	2,884	3,101
Fiduciary assets	12,224	12,214
Intercompany receivables	0	0
Other current assets	389	430
Total Current Assets	16,257	16,382
Goodwill	8,786	8,943
Intangible assets, net	2,844	2,975
Fixed assets, net	816	820
Investments	152	165
Intercompany receivables	0	0
Other non-current assets	1,174	1,201
Investment in subsidiary	0	0
TOTAL ASSETS	30,029	30,486
CURRENT LIABILITIES
Fiduciary liabilities	12,224	12,214
Short-term debt and current portion of long-term debt	802	452
Accounts payable and accrued liabilities	1,440	1,853
Intercompany payables	0	0
Other current liabilities	795	831
Total Current Liabilities	15,261	15,350
Long-term debt	3,770	3,713
Pension, other post-retirement and other post-employment liabilities	2,027	2,276
Intercompany payables	0	0
Other non-current liabilities	1,338	1,342
TOTAL LIABILITIES	22,396	22,681
TOTAL AON SHAREHOLDERS’ EQUITY	7,579	7,762
Noncontrolling interests	54	43
TOTAL EQUITY	7,633	7,805
TOTAL LIABILITIES AND EQUITY	30,029	30,486
Aon plc
CURRENT ASSETS :
Cash and cash equivalents	5	131	17	0
Short-term investments	0	0
Receivables, net	2	5
Fiduciary assets	0	0
Intercompany receivables	23	0
Other current assets	11	7
Total Current Assets	41	143
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	166	166
Other non-current assets	118	117
Investment in subsidiary	10,591	10,398
TOTAL ASSETS	10,916	10,824
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	15	10
Intercompany payables	32	51
Other current liabilities	0	0
Total Current Liabilities	47	61
Long-term debt	194	107
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	3,090	2,890
Other non-current liabilities	6	4
TOTAL LIABILITIES	3,337	3,062
TOTAL AON SHAREHOLDERS’ EQUITY	7,579	7,762
Noncontrolling interests	0	0
TOTAL EQUITY	7,579	7,762
TOTAL LIABILITIES AND EQUITY	10,916	10,824
Aon Corporation
CURRENT ASSETS :
Cash and cash equivalents	251	199	(30)	(21)
Short-term investments	128	89
Receivables, net	2	1
Fiduciary assets	0	0
Intercompany receivables	2,499	2,092
Other current assets	41	53
Total Current Assets	2,921	2,434
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	52	49
Intercompany receivables	2,193	1,997
Other non-current assets	731	735
Investment in subsidiary	10,759	10,522
TOTAL ASSETS	16,656	15,737
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	748	429
Accounts payable and accrued liabilities	76	71
Intercompany payables	3,123	2,637
Other current liabilities	47	49
Total Current Liabilities	3,994	3,186
Long-term debt	2,515	2,515
Pension, other post-retirement and other post-employment liabilities	1,262	1,294
Intercompany payables	166	166
Other non-current liabilities	255	254
TOTAL LIABILITIES	8,192	7,415
TOTAL AON SHAREHOLDERS’ EQUITY	8,464	8,322
Noncontrolling interests	0	0
TOTAL EQUITY	8,464	8,322
TOTAL LIABILITIES AND EQUITY	16,656	15,737
Other Non-Guarantor Subsidiaries
CURRENT ASSETS :
Cash and cash equivalents	152	(39)	336	293
Short-term investments	224	257
Receivables, net	2,880	3,095
Fiduciary assets	12,224	12,214
Intercompany receivables	4,145	3,545
Other current assets	340	370
Total Current Assets	19,965	19,442
Goodwill	8,786	8,943
Intangible assets, net	2,844	2,975
Fixed assets, net	816	820
Investments	100	116
Intercompany receivables	2,108	2,350
Other non-current assets	430	1,174
Investment in subsidiary	8,464	8,322
TOTAL ASSETS	43,513	44,142
CURRENT LIABILITIES
Fiduciary liabilities	12,224	12,214
Short-term debt and current portion of long-term debt	54	23
Accounts payable and accrued liabilities	1,349	1,772
Intercompany payables	2,531	2,162
Other current liabilities	748	779
Total Current Liabilities	16,906	16,950
Long-term debt	1,061	1,091
Pension, other post-retirement and other post-employment liabilities	765	982
Intercompany payables	2,195	2,247
Other non-current liabilities	1,182	1,909
TOTAL LIABILITIES	22,109	23,179
TOTAL AON SHAREHOLDERS’ EQUITY	21,350	20,920
Noncontrolling interests	54	43
TOTAL EQUITY	21,404	20,963
TOTAL LIABILITIES AND EQUITY	43,513	44,142
Consolidating Adjustments
CURRENT ASSETS :
Cash and cash equivalents	0	0	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(6,667)	(5,637)
Other current assets	(3)	0
Total Current Assets	(6,670)	(5,637)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	(4,467)	(4,513)
Other non-current assets	(105)	(825)
Investment in subsidiary	(29,814)	(29,242)
TOTAL ASSETS	(41,056)	(40,217)
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	0	0
Intercompany payables	(5,686)	(4,850)
Other current liabilities	0	3
Total Current Liabilities	(5,686)	(4,847)
Long-term debt	0	0
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(5,451)	(5,303)
Other non-current liabilities	(105)	(825)
TOTAL LIABILITIES	(11,242)	(10,975)
TOTAL AON SHAREHOLDERS’ EQUITY	(29,814)	(29,242)
Noncontrolling interests	0	0
TOTAL EQUITY	(29,814)	(29,242)
TOTAL LIABILITIES AND EQUITY	$ (41,056)	$ (40,217)